In the power industry, the term “electrical grid” is used for an electricity network which includes electricity generation, electric power transmission and electric power distribution.
Electricity generating plants or power farms are usually located away from heavily populated areas and are usually quite large to take advantage of the economies of scale. The electric power which is generated at the power farm is stepped up to a higher voltage, at which it connects to a transmission network. The transmission network moves the generated power long distances, often across a country, and sometimes even across international boundaries, until it reaches its wholesale customer (usually the company that owns the local distribution network). Upon arrival at a substation, the power is stepped down in voltage from a transmission level voltage to a distribution level voltage. As it exits the substation, it enters the distribution wiring. Finally, upon arrival at the service location, the power is stepped down again from the distribution voltage to the required service voltage(s).
A power farm is, typically, a group of power generators in the same location used to produce electric power. A large power farm may consist of several individual power generators, and may cover an extended area of hundreds of square kilometers. An example of a power farm is a wind farm comprising a plurality of wind turbines. The wind turbines, and consequently the wind farm, may also be located offshore and connected to the grid via underwater power transmitting cables.
Utility-scale wind farms must have access to transmission lines of the electrical grid to transport energy. Wind farm developers may be obligated to install extra equipment or control systems in the wind farm to meet the technical standards set by the operator of a transmission line. Such technical standards are often referred to as “grid codes”.
In certain country or region specific grid codes, such as the UK grid code, there are certain performance requirements that need to be met in order for a power park or farm to be allowed to be connected to the power grid (or simply “grid”). For power farms, compliance with the connection requirements is assessed at the interface point of the power farm. Particularly for the interface point between the power farm and the grid, the connection requirements for continuously acting automatic voltage control systems with respect to transient voltage control may be very rigorous.
For an on-load step change in Transmission Interface Point voltage, the continuously acting automatic control system may be required to respond according to the following minimum criteria:                (i) the Reactive Power output response may be required to commence within a predetermined first time period of the application of the step. In some grid codes, such as in the UK, this predetermined first time period is as low as 0.2 seconds. Furthermore, the reactive power output response may be required to progress linearly or vary from a linear characteristic as long as the MVAR seconds delivered at any time up to a second period are at least those that would result from a linear response. In the UK Grid Code, the second period is equal to 1 second.        (ii) the response may be required to be such that, for a sufficiently large step, a first percentage of the full reactive capability needs to be produced within a third period. In the case of the UK grid code, this percentage is 90% and the third period is equal to 1 second.        (iii) the magnitude of the Reactive Power output response produced within a fourth period may be required to vary linearly in proportion to the magnitude of the step change. In the case of the UK grid code, the fourth period is equal to 1 second        (iv) the settling time may be required to be no greater than a fifth period from the application of the step. In the UK grid code the fifth period is equal to 2 seconds.        (v) any change in reactive power and the peak to peak magnitude of any oscillations may be required to be less than a second percentage of the change in steady state Reactive Power within this time. In the UK grid code the second percentage is 5%.        
However, in some cases, communications between the power farm's automatic voltage control system and the local power generator may lag beyond the first time period. This is particularly the case for offshore wind farms where communications may have a non-deterministic character. As a result, the dead-time may be variable and long. As a consequence, if the local power generator waits for a command or signal from the power farm's automatic voltage control system to react, a reactive power demand from the wind farm controller may arrive late to the wind turbine generators and therefore a grid code requirement may not be fulfilled. Furthermore, when local control is used during a first stage and then switched to remote control after a few milliseconds, sudden and uncontrolled reactive jumps may cause instability and excessive oscillations in the wind farm's reactive power, something that goes against voltage step demands of the grid code requirements.
A possible solution would be to improve the communication infrastructure between the power farm's automatic voltage control system and the local power generator controller. However, there are cases, such as in offshore wind farms, where such an improvement may be too expensive or even technically challenging to accomplish, as a substantial portion of the communication lines would need to be underwater.